Swivel wheels in general and castor wheels in particular have the ability to twist their horizontal wheel axis around a vertical swivel axis (regardless of whether that vertical axis passes through or is spaced from the horizontal wheel axis) so as to enable the vehicle or conveyance to which the wheel is attached to change its direction of travel. Such vehicles and conveyances, commonly provided with one or more pairs of swivel castors, find everyday use as trolleys for the transportation, distribution and storage of goods, merchandise and equipment in factories, warehouses, shops and offices and for the movement of patients in hospitals but are most commonly encountered as supermarket trolleys (shopping carts) and luggage trolleys (baggage carts) at airports and railway stations. These latter vehicles tend to be of such a design and construction that a plurality can be nested together to enable a large number to be moved at one time (for example during their collection from dispersed locations and return to a common storage and distribution site) and to create economy of storage space when they are not in use. However, to enable a collection of 20 or so nested trolleys to be moved together, it is essential that all wheels in contact with the ground are able to swivel freely. This in turn means that all four wheels of each trolley must be swivel wheels or that all but one pair of swivel wheels must be lifted clear of the ground as part of the nesting operation.
In general, supermarket and luggage trolleys are provided with four castor wheels. However, such an arrangement has the distinct disadvantage that it permits the trolley to travel in any direction in which the wheels may orient themselves, making steering of such vehicles extremely difficult especially on ramps or other uneven or irregular surfaces, when travelling at any substantial speed or when the castors become worn or fouled with dirt or other foreign material.
In order to overcome this disadvantage, a proposal has been made in European Patent Application No. 0023432 (Fisher) to lock the swivel on the rear wheels into a "straight ahead" position, the lock being released automatically by the act of nesting the trolleys together. However, since the lock operates solely under the action of gravity, there is no convenient means for regaining the castor action when the trolley is in use, it then being necessary to rely totally upon the front castor wheels for steering. This can, of course, cause difficulties when it becomes necessary to maneuver the trolley in confined spaces for example, and seems to destroy the concept of maneuverability for which the castors have been originally utilized.
A somewhat crude and rather unsatisfactory solution to the problem of providing a readily lockable and equally readily unlockable castor swivel has been proposed in U.S. Pat. No. 4,037,291 (Huempfner et al). In the system described a locking and releasing member comprising a shaft and a pair of spaced radially projecting fingers is mounted on the cart borne by the castor wheel so that the fingers can fall under gravity and lie one against each peripheral face of the wheel in its trailing position, the finger surface providing a smooth camming action so that reverse rotation of the wheel lifts the fingers to enable the castor action to be restored. However, the locking and releasing member is mounted on the cart and is not part of the castor wheel assembly The cart manufacturer must therefore include an additional manufacturing step in producing the cart. Also, since the fingers must be spaced apart by the width of the wheel, the locking and releasing member must be matched to the wheel with which it is to cooperate and a replacement wheel will probably not permit proper functioning. Further, if the locking and releasing unit fails, it will almost certainly do so in the locking position thereby destroying all castor action.
A much more sophisticated solution is described in U.S. Pat. No. 4,494,272 (Morita) in which the wheel is mounted in a sub-fork which is rockably pivoted to the main wheel fork, the latter being unable to swivel about the castor axis when the wheel is in forward rotation by virtue of a projection on the sub-fork being engaged by an abutment on the base on which the main fork is mounted. When rotation of the wheel is reversed, a brake operates to stop the wheel, the subfork upon which the brake is mounted rocks to disengage the projection from the abutment and the main fork is free to swivel about the castor axis. This arrangement has the very severe disadvantage that all of the embodiments described involve a very large number of small working parts. This in turn means that the component parts will be expensive to manufacture and assemble and that the complete castor will be considerably less rugged and reliable in operation.